The invention relates to a method for controlling a drive of a motor vehicle having an internal combustion engine and an electric motor (hybrid vehicle), and in particular to a method for controlling a drive of a hybrid vehicle in which the input shaft or the output shaft of the main transmission is connected to the electric motor by means of an intermediate transmission having at least two transmission ratio steps.
A hybrid vehicle having an internal combustion engine and an electric motor, in which the input shaft of the main transmission is connected to the electric motor by means of an intermediate transmission having at least two transmission ratio steps, is known, for example, from German Patent Document DE 198 42 496 A1. The intermediate transmission (or compound transmission) of the electric motor having at least two transmission ratio steps allows the electric motor to work in an optimum way in every operating range of the hybrid vehicle. It is proposed in particular to increase the transmission ratio of the intermediate transmission when a downshift takes place in the main transmission when there is a sudden acceleration demand.
Furthermore, many documents are known which disclose a hybrid vehicle having an internal combustion engine and an electric motor, in which the intermediate transmission between the electric motor and the input shaft of the main transmission has only one transmission ratio step. Various control systems are proposed in this case to obtain as smooth a gear-change as possible and/or as smooth a changeover as possible between the provision of drive by the electric motor and the provision of drive by the internal combustion engine. In most methods, it is ensured that the electric motor or the internal combustion engine is connected to the respective other drive only after the speeds of the electric motor and internal combustion engine have been synchronized and/or that during a shift operation in the main transmission, the speed of the input shaft of the main transmission is regulated or synchronized by the electric motor.
At this point, reference is made by way of example to German Patent documents DE 44 22 554 C1, DE 195 30 231 A1, DE 195 30 233 A1, DE 100 08 344 A1, DE 102 24 189 A1, European Patent documents EP 1 090 792 A2, EP 1 104 712 A2, EP 1 236 603 A2, and United States Patent documents U.S. Pat. No. 6,342,027 B1 and US 2002/0170758 A1.
In contrast thereto, it is the object of the present invention to provide a method for controlling a drive of a hybrid vehicle, in which the input shaft or the output shaft of the main transmission is connected to the electric motor by means of an intermediate transmission which has at least two transmission ratio steps and permits a comfortable shift between the transmission ratio steps of the intermediate transmission of the electric motor.
According to the invention in order to accelerate the motor vehicle from rest, the drive is initially effected solely by the electric motor, the intermediate transmission being in its lowest transmission ratio step, and driving then being taken over by the internal combustion engine before a shift operation in the intermediate transmission.
This method ensures that the internal combustion engine at least partially always takes over the drive function of the main transmission before the intermediate transmission of the electric motor shifts to a higher transmission ratio step, so that a smooth shift is obtained, between the transmission ratio steps of the intermediate transmission in order to provide a comfortable ride for the driver. An interruption in tractive force during a shift operation of the intermediate transmission is thus reliably prevented.
In one preferred embodiment of the invention, the intermediate transmission of the electric motor is embodied as a claw shift transmission. This has the advantage that a relatively simple shift transmission for the electric motor is sufficient, by virtue of the fact that the internal combustion engine takes over the provision of drive torque for the drive in the pause in the shifting of the intermediate transmission.
In one embodiment of the invention, driving is taken over gradually by the internal combustion engine before a shift operation in the intermediate transmission, the drive torque supplied by the internal combustion engine being increased to the same extent as the drive torque supplied by the electric motor is reduced.
In a further preferred embodiment of the invention, driving is taken over by the internal combustion engine as a function of a detectable acceleration demand of the motor vehicle. The acceleration demand of the motor vehicle can be detected in this case, for example, from the accelerator pedal position and/or from the vehicle speed.
In a further embodiment of the invention, an energy store which is connected to the electric motor is intermediately discharged, the electric motor is operated in a regenerative mode, the electric motor is operated in a booster mode and the like only in at least the second transmission ratio step of the intermediate transmission. As a result, the electric motor can be of relatively small and simple design.
If appropriate, the motor vehicle can also be accelerated from rest solely by the internal combustion engine as drive if, for example, the energy store which is connected to the electric motor is discharged to too great an extent, is too cold or overheated.
The features and combinations of features given above, as well as other features and combinations of features, are disclosed in the description and in the drawings. Various specific exemplary embodiments of the invention are illustrated in a simplified manner in the drawings and are described in more detail in the following description.